1. Field of the Invention
The present invention relates to an improved method of operating a regenerative furnace and to regenerative furnaces adapted for said method.
2. Related Art
In the operation of a furnace, the heat balance is highly important. Many processes, installations and devices have been developed to improve the furnace heat balance, in particular by means of the recovery of heat from the flue gas leaving the combustion chamber of the furnace.
An example of such an installation for improving the furnace heat balance is the regenerative furnace, which is widely used in industry, in particular in case of continuously working furnaces.
Regenerative furnaces are equipped with a number of regenerators, each regenerator being connected to the combustion chamber of the furnace via its regenerator port. Said regenerators operate alternately as (1) heat-recovery device for the recovery of heat from the flue gas leaving the combustion chamber of the furnace and as (2) pre-heater for preheating the combustion oxidant, typically combustion air, fed to the combustion chamber. This is achieved by means of refractory material present in each of the regenerators.
When a regenerator operates as heat-recovery device, i.e. when the regenerator is in a heat-recovery mode, hot flue gas from the combustion zone enters the regenerator via its regenerator port.
The hot flue gas flows through the regenerator, thereby heating the refractory material present in the regenerator, and leaves the regenerator as cool flue gas via the regenerator outlet.
When a regenerator operates as pre-heater, i.e. when the regenerator is in a pre-heating mode, cool combustion oxidant enters the regenerator via the regenerator inlet. The cool combustion oxidant flows through the regenerator and is heated by heat exchange with the refractory material present in the regenerator, which refractory material has been heated in an earlier heat-recovery mode of the regenerator. Pre-heated or “hot” combustion oxidant is then supplied by the regenerator or regenerators operating in pre-heating mode, and combusts, or in other words burns, with fuel injected in the combustion chamber by one or more burners or fuel injectors.
Said burners or fuel injectors are generally situated in the vicinity of the regenerator ports, typically underneath the individual regenerator ports.    (a) When the temperature of the cool flue gas leaving the regenerator or regenerators operating in heat-recovery mode reaches a predetermined upper limit;    (b) when the temperature of the hot combustion oxidant leaving the regenerator or regenerators operating in pre-heating mode reaches a predetermined lower limit; or    (c) when the duration of the heat-recovery mode or of the pre-heating mode of a given regenerator or set of regenerators reaches a predetermined length of time,the operation of the regenerators is reversed:            the regenerator or regenerators which have been operating in heat-recovery mode are switched to operation in pre-heating mode as described above, and        the regenerator or set of regenerators which have been operating in pre-heating mode are switched to operation in heat-recovery mode as described above.        
In this manner, an improved energy balance is maintained throughout the operation of the furnaces.
Such a process is also known as a reverse firing process or operation.
It is a problem with known regenerative furnaces that, immediately following the reversal of the operation of the regenerators, no combustion oxidant is in fact supplied by the regenerators to the combustion chamber. Indeed, on the one hand, (1) the regenerator or set of regenerators which before the reversal were in the pre-heating mode, no longer supply combustion oxidant to the combustion chamber and instead receive flue gas from the combustion chamber, whereas, on the other hand, (2) before supplying hot combustion oxidant to the combustion chamber, the regenerator or set of regenerators, which before the operation reversal were in the heat-recovery mode, supply, during a limited transition period, to the combustion zone flue gas which filled or was present inside this regenerator or set of regenerators before the operation reversal. This particular stage in the operation of a regenerative furnace is hereafter referred to as “transition phase”.
As the flue gas injected into the combustion chamber during the transition phase cannot adequately sustain combustion in the combustion chamber, no fuel is normally injected into the combustion chamber during this phase. The heat generation in the combustion chamber is thus reduced or even interrupted during said transition phase every time the operation of the regenerators is reversed, with a corresponding reduction in the productivity of the furnace.
For example, in a continuous regenerative glass-melting furnace with a pull rate of 50000 kg glass per day, in which reversal of the operation of the regenerators takes place every 17 to 22 minutes with a transition phase of 40 to 60 seconds per operation reversal, heat generation in the combustion chamber is reduced or interrupted during 3 to 5% of the overall furnace operation time with a corresponding negative effect on the pull rate of the furnace.